The cells formed in the cambial zone are elongated and lignified under restraint of the matured cells in the stem. It may be considered that the growth stress in wood arises from this restraint. This note attempts to determine the growth stress distribution in the stem, in which the mechanical constants, such as the Young's modulus, Poisson's ratio and the modulus of rigidity, are distributed with some regularity. The stress distribution was calculated by a Finite Element Method. The displacement of each element was strongly dependent on the restraint of bark. From the comparison of the calculated displacements with the experimental values, it appears that the growth stress in wood would be the accumulation of the stress history in it.
In order to make clear the relationship between the internal stress developed in paint films of finished wood and their mechanical properties, the strength properties and the stress relaxation in tension of paint films for wood coating were measured. The paints used in the experiments were polyester, polyurethane, amino alkyd resin and nitrocellulose lacquer (Table I). The results obtained are as follows: (1) The Young's modulus, proportional limit and breaking stress of polyester and amino alkyd paint films were greater than those of polyurethane and lacquer paints, while the breaking strain of the former was smaller than that of the latter (Table II). (2) In the stress-strain curve of polyurethane paint film, the yield point was recognized in all cases, and the plastic deformation was considerable above the yield point (Fig. 1). (3) The strain at proportional limit of paint films was a constant value of 0.4∼0.9% without regard to the temperature (Table II). (4) The Young's modulus, proportional limit, breaking stress and breaking strain of paint films changed almost linearly with increasing temperature (Fig. 2). Their temperature coefficients were as shown in Table III. (5) The order of the magnitude in the relaxation of stress was as follows: PU>NC>AA≈PE, as shown in Fig. 4 and Table IV. (6) In the case of stress level of 10, 30, and 50% of the proportional limit, plots of relaxation coefficient (m) against stress level showed approximately a linear relationship (Fig. 5). (7) The results in the long-duration stress relaxation test showed that the stress in polyester paint film decreased about 15% of initial stress in 10hrs., the amino alkyd paint film about 52% in 33.3hrs., the lacquer paint film about 64% in 21.6hrs., and the polyurethane paint film about 86% in 30hrs. (Table V).
This experiments was confined to consideration of the segregation mechanism taking place when a binary mixture of wood particles of different sizes was vibrated in a vessel under various conditions. The coefficient of segregation was defined by representing how much fine particles percolate through a known amount of coarse particles. After the rapid size-segregation of wood particles in the early stage of vibration, the rate of segregation gradually decreases due to the saturation of fine particles in the passages among the coarse particles and to the mutual interference between the fine and the coarse particles. Generally speaking, the size-segregation is enhanced markedly with higher centrifugal effect due to the increase in the intensity of vibration. However, the wood particle layers do not segregate under extremely low frequencies or amplitudes, in spite of the theory which shows the increase of centrifugal effect with increasing either frequency or amplitude. The size-segregation becomes slower as a result of the mixing behavior when the cycle of bouncing of particles becomes out of harmony with the cycle of vibration of bed under the condition of extremely high frequency and amplitude of vibration.
In the previous report, the authors gave the following clear physical interpretation for the process of creation of residual stress in wood log: The distribution of residual stress in a tree is set by the superposition of growth stress on the outer layer of the tree. However, the precise measurement of residual stress in a tree trunk must be needed for estimating the mechanism of occurrence of growth stress from residual stress in a tree. Now, the experimental values for growth stress reported in literature are not residual stresses in a tree trunk but one-dimensional recovery strains. Therefore, the past data cannot be used for such a purpose. In the present study, the residual stresses in tree trunks have been measured by“thin layer removal method”proposed-by Doi & Kataoka. The experimental results are shown as in Fig. 1. The tendency of residual stress distribution in both longitudinal and radial directions is similar to that of the past data. But, only in the tangential direction, the compressive stress appeared near the center of tree trunk. The residual stress in the tree measured by this method can be used for estimation of the mechanism of occurrence of growth stress in a tree.
In this study, the physical and mechanical properties of cell wall of newly formed compression wood of Japanese black pine (Pinus Thubergii Parl.) have been investigated. The results obtained are summarized as follows: (1) When the compression wood was formed in a tree, a large compressive stress resulting from growth stress appeared at the tissue near cambium. Its compressive stress level was about 10% of the tensile strength. (2) The tensile strength of the compression wood was smaller than that of normal wood or cell wall near cambium. (3) The relaxation quantity (σt/σ0) of compression wood was larger than that of normal wood. (4) The relaxation quantity in the compression wood decreased as the lignification of cell wall progressed. (5) The restrained dry wood under tensile stress (about 20% of tensile strength) showed a different cell wall condition of tracheid than the unrestrained dry wood. The most pronounced difference appeared in their shapes of pits on the radial section.
The purpose of this study is to reveal the fracture process of the single coniferous tracheids under tensile stress and the mechanical behaviour of two kinds of bordered pit, i.e. the cross field pit and the intertracheal pit. The early and latewood single tracheids prepared from the delignified wood chips of SUGI (Cryptomeria japonica D. Don) were freeze-dried, and mounted on the specimen tabs. The single tracheids were stretched after setting on the tensile device. The behaviour of the single tracheid in stretching within the SEM column was observed with a monitor television and recorded on a video tape recorder. Furthermore, the broken ends of the single tracheids were investigated in detail using the SEM. The results obtained from these observations are as follows; The initial crack of the single tracheid almost always took place at the cross field pit near the end of ray crossing (Figs. 3-2, 5-1, 6-2 and 7). In earlywood tracheids, the initial crack propagated across the cell wall of the ray crossing, accompanying a twist around the cell axis (Fig.3-2). The broken end of fractured S2 layer caused by the initial crack propagation showed the minute splintery mode, indicated by an arrow in Fig. 5-2, because the S2 microfibrils should have been severed with some slippage among them. Furthermore, the crack sometimes propagated longitudinally along the cell corner (Fig. 4). The initial crack of the latewood tracheid propagated almost perpendicular to the cell axis, and the initially fractured surface of the S2 layer was very smooth (Fig. 7). In both the early and latewood single tracheids, the fractured wall occurred at the final stage of fracture process showed conspicuously the long splintery mode (Figs. 3-6 and 6-4), indicating the intermicrofibril slippage. The initial crack scarcely occurred at the intertracheal bordered pit. In earlywood tracheids, the intertracheal pit makes resistance to crack propagation in the cell wall, and so the crack ran around the pit margin (Fig. 10). In latewood tracheids, the pit border split along the S2 microfibril orientation, but the layer in which the microfibrils oriented concentrically was unbroken (Fig. 11).
In order to make clear the bark-water relationship, the extractive content and the heat of wetting at 25°C by means of a twin-type conduction microcalorimeter were determined on the barks of 22 different American hardwoods growing on southern pine sites. Furthermore, sorption isotherms were determined at 20°C and 30°C for birch wood and bark, using a vacuum sorption apparatus of high sensitivity. The results obtained are as follows: (1) Hardwood bark is higher in extraneous substances and lignin contents, and lower in holocellulose than wood. Especially the tendency is remarkable for outer bark. (2) The sorption isotherms at both temperatures for inner bark of birch were almost in agreement with those of wood, but the curves for outer bark were much lower. (3) The total heat of wetting for inner barks of 22 species ranges between 14.16 to 20.45 calories per gram of dry bark. (4) From the results of the sorption isotherms and the total heat of wetting, it seems that the inner bark-water relationship is similar to the wood-water relationship, but the outer bark-water relationship is inferior to that of inner bark or wood. It can be considered that these tendencies are closely related to the amounts of chemical components in wood and bark.
By taking account of the fine structures and the proportion and distribution of the chemical constituents, the values of dielectric constant for the tracheids in early and late woods as well as for the ray cell were theoretically calculated. Furthermore, by considering the fraction of cell wall and the arrangement of cells, the values of dielectric constant for early and late woods and those for wood in three principal directions were also calculated. From the results, it has been found that the anisotropy of dielectric constant of wood in the transverse direction depends on the fraction of late wood and the value of dielectric constant in the tangential direction is influenced remarkably by the irregular arrangement of cells. The experimental values almost coincided with the theoretical ones.
The effect of the anatomical structure on the relaxation modulus of wood as a porous anisotropic material was discussed experimentally and theoretically through the numerical value of form exponent n (Eq. 1). Stress relaxation tests were carried out on a simplified model of the cell-lumen arrangement in the cross section of wood (Fig. 1) and the numerical values of n at 10, 102, 103sec. were determined. On the basis of these experimental data, radial and tangential relaxation moduli were calculated by considering wood as a complex laminate with two stages of lamination, i. e. a laminate of alternating layers of transversely anisotropic earlywood and latewood perpendicular to ray tissue (Fig. 3). In this paper, a“strength of materials' approach”was used in order to compute the relaxation modulus of the above complex laminate. Modulus of ray tissue was also calculated theoretically. The results obtained are shown in Figs. 2 and 4. It was found that; (1) The viscoelastic anisotropy of wood was strongly dependent upon the cell-lumen arrangement. However, in the viscoelastic analysis of whole wood, a unique complex lamination for wood must be taken into consideration in addition to cell geometry. (2) As the numerical value of n was independent of time and the simplest mixture rule was applicable to estimate the modulus of wood, it might be considered that the relaxation process of wood was due to that of wood substance.
Setting in wood plays an important role in the practice of wood drying. But its role is not well established and the true nature of setting in wood is problematical. Presently, the most important matter is to clarify the fundamental, physical, mechanical and Theological properties of set wood. In order to clarify the rheological properties of set wood, the effect of tensile stress applied during drying on the Young's modulus of wood was determined by means of the vibrating reed method (70-85Hz) and the longitudinal resonance method (7-8kHz). The Young's modulus of set wood increased in proportion to the degree of set and the rate of increase against normal wood is in the order of 1.12-1.20. On the other hand, tan δ of set wood does not depend clearly on the stress applied during drying. Then, using the vibrating reed method, the change of Young's modulus of set wood during moisture-recovery of set was determined. The result confirms that the moisture-recovered wood loses its past history of the stress applied during drying. Based on this fact, the moisture-recovery of set causes not only the recovery of apparent dimension of a specimen but also the recovery of some structure formed by the stress applied during drying. These characteristic natures of set may be attributable to some reversibly changeable structure of wood in the microscopic level.
Wood was plasticized with several solvents for cellulose and lignin, i. e. dimethylsulphoxide (DMSO), and DMSO solutions of SO2 (SO2-DMSO), of SO2-diethylamine complex (SO2-DEA-DMSO), and of N2O4 (N2O4-DMSO), and the plasticized wood specimens were set under a constant deformation condition by displacing the solvents with water followed by drying. The residual stress in the plasticized wood specimens under torsional deformation and the elastic modulus of torsion were measured during the displacement and drying process. Furthermore, the residual deformation in bending after successive steps of the process and the springback of the set specimens in water were determined. The most effective setting was obtained by the treatment with N2O4-DMSO. A highly effective setting, compared with the drying-set of untreated wood, was also obtained by the treatment with other solvents. The viscosity measurement of cellulose and lignin solutions in these solvents and X-ray study of the treated wood suggest that the decrystallization of wood with N2O4-DMSO and the swelling of lignin in wood with the solvents promote the flow of the network constituting wood substances and that the successive rearrangement of the network during the removal of solvents with water and subsequent drying fixes the deformation of the specimens.
To investigate the service life of wood-based bearing wall panels for exterior use, simple test methods of rigidity and horizontal shear strength of the semi-scale models of panels are being developed by the authors' group. One of the most consistent and reliable test methods for repeated rigidity measurements before and after outdoor exposure has been established, utilizing the diagonal compression of two square panels with frames on the edges (Figs. 1 and 2). The sizes of the panel shown in Fig. 5 are recommended from the feasibility. The results and discussions are summarized as follows: (1) Consistency and invariability of the new method after the repeated loads are verified. (2) As for the glued or nail-glued panels, the rigidity and strength of five kinds of wood- based panels measured by this method are 1.5 to 3 times larger than those by the traditional racking test method, which is mainly due to the difference in stress distribution and load transmission between two methods. (3) The stress distribution in the facing material of the panel loaded by this method is more homogeneous and symmetric than that by the racking test method. (4) The load transmission along the edges of a panel loaded by this method is rather insufficient in comparison with that in the racking test. (5) As for the nailed panels, loosing of nails causes the stress distribution to become even, so that almost the same strength is attained by the two different test methods.
To make clear the real buckling behaviour, the fundamental equations (I-21 and 22) were developed for the analysis of the finite deformation of plywood with imperfections-Eqs. 1 and 2 are the simple forms of the fundamental equation which does not include the distribution of the Young's moduli in xy-plane. The author made the computer program (in FORTRAN IV) to solve these equations by means of the non-linear finite difference method. Accuracy of the solution and the computation results-influences of the initial deflection (geometrical initial imperfection) and the distribution of the Young's moduli-were shown in the previous paper. In this paper the influence of the eccentric load on the load-deflection (L-D) curve of the plywood with simply supported four edges (Fig. 1) and the influence of the unsymmetrical lamination (Figs. 2 and 3) and the effect of the combination of the imperfections (Fig. 4) were investigated. From these figures the following conclusion is obtained: -The typical buckling occurs only in the special cases; (i) perfect plywood and perfect loading, and (ii) counterbalance of the imperfections. Therefore, the so called“buckling load”is not observed clearly in general. And the buckling behaviour disappears according as the imperfection becomes large. The influence of initial compression on the L-D curve of the plywood under shear is also observed (Figs. 5 and 6). When the initial compression is not too large, the L-D curve resembles that of the plywood which has the same initial deflection as the deflection caused by the initial compression. So the initial compression can also be assumed one of the imperfections. The influence of the distribution pattern of the external shear load is shown in Figs. 6 and 7. The plywood whose face grain is inclined 45° to the horizontal edges (45°-plywood) is affected more than that of 90°(90°-plywood). The effect of the combination of the clamped edge (only the edge angle is fixed) and the simply supported one (roller support) is shown in Fig. 11, and this figure shows that the 45°-plywood exhibits the buckling behaviour at lower shear load than the 90°-plywood when more than one edge is clamped. The intermediate edge condition of“simply”and “clamp” can be expressed by“s”of Eq. 3. The bigger the support factor“s”becomes, the higher the L. E.B.B. (load at entering the buckling behaviour) becomes. But the L.E.B.B. of the 45°-plywood does not increase so much as that of the 90°-plywood, and“s=0.5”is nearer to the“clamp”than to the“simply”(Fig. 11).
In order to obtain some informations concerning the effect of vibration in vibratory cutting of wood, the coefficient of static friction under vibration was investigated. The experiments were carried out using a sample of wood block and as an antisample, a brass plate vibrating with low frequency 20-100Hz. The coefficient of static friction under vibration could apparently be reduced. And the greater the acceleration of vibration, the more reduction of the apparent coefficient of static friction was obtained. Furthermore, the experimental results were in good agreement with the theoretical results. Under a sinusoidal vibration with one degree of freedom, the apparent coefficients of static friction in the three directions, μx (parallel to the direction of vibration), μy (perpendicular to the direction of vibration), μπ/4 (45°-direction to the direction of vibration), are given by the following formulas respectively, μx=μ0-λ, μy=√μ02-λ2, μπ/4=√μ02-λ2/2-λ/√2 where μ0 is the true coefficient of static friction and λ=aω2/g (a=amplitude, ω=angular frequency, g=acceleration of gravity) is the ratio of accelerations. Under three-dimensional vibration, furthermore, the apparent coefficient of static friction in the y-direction, μy, is given by the following formula, μy=√μ02(g+z)2-x2-y/g where x, y and z are the acceleration of vibration in the x-, y- and z-direction respectively.
In an effort to evaluate the synthetic wood pulp (SWP) derived from high-density polyethylene as a paper-making material, the porosity properties of the handsheets prepared from SWP only and from the blends of SWP and softwood kraft pulp were studied. Scanning electron micrographs show that the SWP fiber consists of partially branched strands or fibrils. This also indicates why SWP fibers have a large number of the small pores on the fiber surface and within the fiber. It was estimated from the mercury penetration curve of SWP handsheet that the volume of the small pores less than ca. 0.5μ in equivalent pore radius (ca. 14kg/cm2 in mercury penetration pressure) corresponded to about 37 percent of the volume of SWP fiber component. The blend of SWP fibers having such morphological properties leads to a change in porosity properties of blend-handsheets as well as an improvement in optical properties. The total mercury penetration volume (porosity) increased remarkably with increasing the SWP pulp content. On the other hand, the mercury pressure of blend-handsheets at which maximum penetration occurred was higher than that of wood pulp handsheet. This fact seems to indicate that the neck of interconnected channels within the blend-handsheets becomes narrower than that in wood pulp handsheet in disregard of increase in porosity. By heat-pressing under a pressure of 3.5kg/cm2 at temperatures under 130°C, the porosity of blend-handsheet decreased remarkably without reducing the volume of the small pores of SWP fibers. In addition, porosity properties were studied by the air permeation method, and it was found that the permeability coefficient increased with the higher SWP content as a result of increase in number of channels in blend-handsheets.